I use a Pentax K5 which has a similar(same?) sensor as the 5R. I use ISOs up to 3200, the shot below which is lit entirely by firelight is using a pentax 16-45 zoom, the second one using the Sigma 30mm f1.4 For both use apertures less than f1.4 purely to give me a bit more DOF I always brace myself against something if I can, or use a monopod. Your initial guess on the sort of exposures needed to capture firelight isn't far off....